The present invention generally relates to optical information storage units, and more particularly to an optical information storage unit for reproducing a magneto-optic signal recorded on both a land and a groove of a magneto-optic recording medium.
In this specification, an "information storage unit" refers to an apparatus which records information on and/or reproduces information from a recording medium.
Presently, optical recording mediums are popularly used as recording mediums capable of enabling reproduction of audio signals and image signals therefrom. Particularly, active research and development are made with respect to magneto-optic recording mediums and phase change recording mediums, as rewritable high-density recording mediums.
Two techniques are conceivable in order to improve the recording density of the optical recording medium which records information spirally or concentrically, namely, reducing the track pitch and improving the linear recording density. In either case, the recording density can be improved by shortening the output wavelength of a semiconductor laser which is used for the recording and reproduction. However, a green or blue semiconductor laser which produces an output having a short wavelength and can continuously oscillate with stability at room temperature is unlikely to be put on the market at a low cost for still some considerable time. Under such circumstances, there are demands to realize a method of greatly improving the recording density by using the laser which produces an output having the presently available wavelength, such as a magnetic super resolution (MSR) proposed for the magneto-optic recording medium.
In the case of a RAM medium such as the magneto-optic recording medium, light having the same wavelength is used both at the time of recording information and at the time of reproducing information. On the other hand, in the case of a ROM medium which is prerecorded with information, phase pits are formed using a gas laser or the like which produces an output having a short wavelength. Hence, the reproducing conditions are the same for the RAM medium and the ROM medium, but the RAM medium is disadvantageous compared to the ROM medium from the point of view of recording the information with a high density, because the recording of the RAM medium requires a light source which will only be available in the future. For this reason, in the DVD standard to which much attention is drawn as a next-generation video recording medium for home use, no proposal has yet been made which would support the recording capacity of the ROM medium on the RAM medium having the same medium size as the ROM medium.
If the same linear recording density and the same track pitch are used, it is possible to simply double the recording density by recording the information on both the land and the groove as compared to the case where the information is recorded only on one of the land and the groove. Hence, the method of recording the information on both the land and the groove is an extremely important method from the point of view of developing a high-density recording medium. It has been reported for the magneto-optic recording medium that the MSR described above can not only improve the linear recording density but also reduce crosstalk between the tracks, and various studies are being made on the possibility of applying the MSR to the information recording on both the land and the groove. However, the condition for realizing the MSR is complicated in that, for example, the reproducing laser power depends upon the linear velocity, a reproducing magnetic field is required in some cases, and at least three magnetic layers are required in some cases. Therefore, the MSR is uncertain as to the stability, and the realization is likely to increase the cost of the apparatus.
When the track pitch is reduced, the crosstalk caused by the mixing of a data signal from the adjacent region into the output signal becomes a problem. According to the conventional method which records the information on the land or records the information on the groove, the groove exists between two adjacent recorded lands or, the land exists between two adjacent recorded grooves. As a result, the adjacent regions recorded with the information are separated by a groove or a land, thereby suppressing the crosstalk. However, when recording the information on both the land and the groove, the regions recorded with the information are adjacent to each other, and the effect of the crosstalk on the reproducing characteristic is extremely large. A Japanese Laid-Open Patent Application No. 8-7353 proposes selection of the depth of the groove so as to reduce the crosstalk from the land or the groove. According to this proposed method, in a case where the wavelength of the light is 680 nm, the numerical aperture of an objective lens used is 0.55 and the widths of the land and the groove are 0.7 .mu.m, the crosstalk is reduced by setting the depth of the groove to approximately 1/6 the wavelength.
But even if the spot size of the light irradiated on the magneto-optic recording medium is the same, as the track pitch becomes narrower, there was a problem in that the crosstalk cannot be reduced sufficiently in the case of a groove having the depth on the order of approximately 1/6 the wavelength. In addition, compared to the normal case where the depth of the groove of the magneto-optic recording medium is 1/8 the wavelength, there also were problems in that a carrier level of the signal decreases and a level of a push-pull signal which is used as a tracking error signal also decreases in the case of the groove having the depth on the order of approximately 1/6 the wavelength.
On the other hand, it has been reported that the condition for reducing the crosstalk in the proposed method, that is, the setting of the depth of the groove of the magneto-optic recording medium, easily changes depending on the Kerr ellipticity, spherical aberration and focus error of the objective lens, and the like.